This invention relates to a method of providing d.c. feed from a line interface circuit to a two-wire telephone line. As is well known, d.c. feed is one of a variety of functions which must be performed by a line interface circuit provided at a telephone central office (CO) or remote terminal, and includes the supply of a direct current, generally referred to as loop current, to a telephone line for purposes such as determining the hook state and dial pulsing of a telephone connected to the line and supplying a bias current or power to the telephone.
It is common to provide this current from the CO battery supply via balanced feed resistors. Because the line impedance may vary significantly for lines of different lengths, and because the battery voltage is relatively constant, the loop current also can vary widely and, in particular, can be much higher for short lines than is required for proper operation of the telephone equipment. This results in the disadvantages of undesirably high power consumption and dissipation, especially in the feed resistors.
Although constant current d.c. feed arrangements have been proposed in order to avoid these disadvantages, these give rise to other disadvantages. Accordingly, a need has remained for a voltage driven d.c. feed arrangement in which the above disadvantages are reduced.
This need is addressed in Treiber U.S. Pat. No. 4,254,305 issued Mar. 3, 1981 and entitled "Current Limited Subscriber Line Feed Circuit", in which loop current on a telephone line is sensed and used to control a voltage generator, whose output is coupled to the line via the feed resistors, in order to reduce the supply voltage when the loop current exceeds a threshold level of 40 mA. In this circuit, voice signals are coupled to and from the line via a conventional transformer and electronic hybrid circuit.
With evolution of telephone line interface circuits, it has been recognized that it is desirable to reduce size, cost, and power consumption, and to increase versatility, as much as possible. This involves eliminating bulky conventional transformers, using common line drive circuitry for d.c. feed and a.c. signal handling, and programming components such as digital signal processors and control circuitry to enable a single line interface circuit to be adaptable to many different telephone line characteristics and telephone service capabilities. With such evolution, the line feed circuit arrangement of the Treiber patent discussed above is not practical.
In Chea, Jr. U.S. Pat. No. 4,315,106 issued Feb. 9, 1982 and entitled "Apparatus For Regulating Current Supplied To A Telephone Line Signal Of The Type Employed In Digital Telephone Systems" there is described an arrangement in which loop current is regulated by a differential amplifier and summing arrangement whose output is used to control a voltage amplifier, which may be constituted by a d.c. to d.c. converter, to produce a supply voltage for the line. This patent does not address the coupling of signals to and from the line.
In the related Chea, Jr. U.S. Pat. No. 4,317,963 issued Mar. 2, 1982 and entitled "Subscriber Line Interface Circuit Utilizing Impedance Synthesizer And Shared Voltage Source For Loop Current Regulation Control" the supply voltage is described as being switched among a plurality of voltage sources for loop current regulation, and signals are coupled to the line via amplifiers which with their surrounding components are described as comprising "a pair of buffer amplifiers capable of handling the dc and ac electrical requirements of the SLIC function". There is no further description as to the extent of these requirements or how they would be met.
There are many such requirements of the line interface circuit function which are well known in the art, and which are both stringent and difficult to meet in a versatile and adaptable manner. One of these requirements is to permit reversal of the polarity of the supply voltage to the telephone line. Generally, this requirement has been met by providing a polarity reversing relay via which the line interface circuitry is coupled to the line, but the reversing relay is a mechanical component of the line interface circuit, constituting part of its bulk and cost and being subject to mechanical failure. It is accordingly desirable to avoid the need for this reversing relay while still meeting the line interface circuit requirements. However, the latter Chea, Jr. patent would require that a reversing relay be present to permit polarity reversal.
In Bolus et al. U.S. Pat. No. 4,431,868 issued Feb. 14, 1984 and entitled "Solid State Telephone Line Interface Circuit With Ringing Capability" there is described a line interface circuit in which both d.c. and a.c. signals are supplied to the inputs of high-voltage tip and ring wire amplifiers whose outputs are coupled to the fine. The d.c. levels at the amplifier outputs are determined by d.c. potentials at the amplifier inputs and the d.c. gain of the amplifiers. The description states that it is possible for the tip and ting d.c. polarities to be reversed by reversing the polarities at the inputs to the amplifiers, but does not discuss this further.
This patent is concerned with using the amplifiers also to produce a (high voltage) ringing signal waveform on the line, as well as to couple voice signals to the line. To these ends, and to ensure operation of the amplifiers within linear parts of their characteristics, the supply voltage rails of the amplifiers are controlled to follow, with an offset, the instantaneous values of the ringing signal waveform or d.c. feed characteristics which are not described in detail. The supply voltage rails are coupled to outputs of a d.c. to d.c. converter which is controlled by a sensing circuit and error amplifier in dependence upon the supply voltages and the amplifier output voltages.
In the Bolus et al. patent the two amplifiers must be operated in a high-gain configuration, thus requiring that the gain-bandwidth product of the amplifiers be much larger than the bandwidth required to support telephony signals. In addition, the amplifiers must accommodate the high ringing voltages, and therefore must be fabricated using a very high voltage silicon process. This combination of requirements makes the approach of the Bolus et al. patent impractical. To carry a variety of voice and data traffic, including ISDN signals, requires an even larger bandwidth, and also adds a requirement for a fast slew rate. The approach of the Bolus et al. patent is also impractical for meeting these additional requirements.
An object of this invention is to provide an improved method of providing d.c. feed to a two-wire telephone fine.